What happened to Bonnie?

Cardiac arrest was more than likely the cause of death.

Bonnie was morbidly obese. She was also a smoker. These 2 factors dramatically increase your chances of developing heart disease.

Bonnie rarely moves off of the couch. She gets up to use the restroom. That's the only physical activity that she does. When you are that overweight and aren't used to any physical activity, you can definitely have a heart attack after suddenly climbing a flight of stairs.

We all know that a regular exercise program is good for us, but it is important to work up to a level of fitness and not just “jump in.” If you are not used to regular aerobic exercise, sudden and strenuous physical exertion can lead to a heart attack. This can include everything from playing a competitive game of basketball with friends to going hunting and carrying an animal. Too much exertion could also come from sex with a new partner/sexual activity, running or shoveling snow.

“You should avoid being over strenuous in activities such as these if you are not used to exercising, have cardiac risk factors such as a family history of heart disease, diabetes, high cholesterol or high blood pressure, for starters. Testing your ability to exercise, especially in weather extremes, can be a dangerous proposition,” says Curtis Rimmerman, MD, of Cardiovascular Medicine at Cleveland Clinic.

I believe that Bonnie actually went upstairs to lay in her own bed and die. I believe that she knew that climbing those stairs would kill her and she wanted to die in that manner, so she basically chose her own death bed.

I always thought maybe she had taken some medication to induce an overdose or something based on the conversation with Gilbert before her death. Arnie found both parents dead. Such a sad moment in the movie.